1. Field of the Invention
This invention relates to relaxation type oscillators incorporating resistive and capacitive elements (RC) and, more particularly, to a RC relaxation type oscillator circuits suitable to be fabricated in integrated circuit form to which a resistive and capacitive frequency determining network is coupled thereto at a single external pin of the oscillator, and wherein the frequency of oscillation is made substantially independent to process and temperature variations in the integrated oscillator circuit.
2. Description of the Prior Art
Relaxation oscillators which generally rely on resistive-capacitive frequency determining networks are utilized in many applications including timing circuits in data transmission systems, sweep oscillators, television receivers, reference oscillators in decoders for stereo FM radio receivers, telephone ringers and telecommunication systems, and the like. Moreover, differential relaxation oscillators that are suited to be fabricated in integrated circuit form are known in the art. For example, U.S. Pat. Nos. 3,688,220 and 3,824,494 disclose such oscillators wherein a sawtooth oscillation signal is generated having a frequency of oscillation determined by the RC frequency determining network.
As disclosed in the prior art relaxation oscillators are found in many different forms but generally are quite sensitive to supply voltage variations, temperature variations and the charge rate at which the capacitive element is charged and discharged which determines the oscillator frequency period. In one favored form the prior art relaxation oscillators comprise a differential comparator amplifier which has a reference voltage applied to one input thereof and the frequency determining network coupled to the other input. In general operation of this type of relaxation oscillator the capacitive element of the frequency determining network is charged at some rate until such a time that the voltage developed thereacross exceeds the trip point of the comparator at which time the capacitor is then caused to be discharged through the resistive element of the frequency determining network. One of the main problems that arise in some of the prior art integrated circuit relaxation oscillators including a comparator amplifier is due to the inaccuracies associated with the charging rate of the capacitor, i.e., the percentage of the oscillation time period that is required to charge the capacitor to its peak value. How accurately the capacitor is charged also affects the precision of the oscillation frequency. For instance, if the rate of charge of the capacitor is too slow the accuracies of the oscillator circuit are degenerated since the rate of charge then becomes a function of the devices of the integrated circuit, i.e., the rate will depend on the beta amplification factors of the transistor as well as the junction resistances thereof which vary both with temperature and process variations. Hence, the frequency of oscillation will also vary. If the rate of charge of the capacitor is too fast, the response time of the comparator amplifier becomes critical. The desired charge on the capacitor can be exceeded if the differential amplifier cannot switch at the exact moment the voltage across the capacitor exceeds the reference voltage level as aforedescribed. This causes overshoot of the desired peak voltage across the capacitor which affects the operating frequency of the oscillator. Therefore, due to these errors, it has been very difficult, if not impossible, to fabricate precision differential relaxation oscillators in integrated circuit form.
Thus, a need for a precision relaxation oscillator suited for fabricated in integrated circuit form having an accuracy of .+-.5% of the desired operating frequency using 1% standard external components arises.